E-ON Netz has produced a report on the problems associated with the fluctuating availability of wind energy.
E.ON Netz corporation controls a high-capacity transmission grid that reaches from Scandinavia to the Austrian Alps. 
"It oversees 32,500 kilometres of high-voltage and extra-high voltage lines in Germany, covering approximately one third of the country, and is one of the largest electricity grid operators in Europe. In the UK, E.ON Group owns Powergen.
"Within E.ON Netz's control area there is 6,250 MW of wind power, which makes it one of the world's most experienced companies in regard to the difficulties of integrating a stochastic power such as wind generated electricity into a stable grid. (See Wind Report, p. 15 for further details.) The Wind Report 2004 provides data for 2003, and can be downloaded (in PDF format - About 350Kb size)"